A wide variety of sealing means have previously been provided for valves. In those valves designed for corrosive and toxic chemicals, such as those found in tank trailers, the design parameters are particularly stringent, since even minor leakage should be avoided. A number of such aggressive chemicals are now pumped under pressure into tanks from the bottom, in order to prevent escape of emissions from a top loading port on a tank. Such procedures, however, further complicate the stresses on the valves, since bottom loading significantly increases the pressures to which the valve sealing surfaces are exposed, and subjects the valves to stresses for which they were not originally designed.